Condenser microphone



May 7, 1940.

R. HORKHEIMER CONDENSER MICROPHONE Filed Nov. 7, 1956 INVENTOR ROLF HO IHEIMER BY I ATTORN EY Patented May 7, 1940 UNlTED STATES CONDENSERmanor-none RolfHorkhimer, Berlin, Germany, assignor, by

mesne assignmentato Radio Gorporati n of America, New York, N. Y., acorporati n of Delaware Application November I, 1936, Serial No. 109,624

Germany November 21, 1935 5 Claims. (c ne-111) This invention relates toimprovements in or relating to electrostatic microphones of satisfactoryefficiency. I

7 It is well known to use very thin and light 6 diaphragms for themanufacture of electrostatic microphones of satisfactory efficiency.Generally, such diaphragms' are annularly glued at their circumferenceinmore orless stressed position or maintained in positioned andpreviously 10 stressed by means of particular devices. The aircushionsituated behind the diaphragm serves substantially as the force -ofreaction. Thus, a high starting pressure is obtained due to thesubstantial increase in capacityof the microphone;-

v The capacity'may, :be increased by reduction of the spacing betweenthe diaphragm and the counter electrode but this reduction is limited bythe capability of resisting puncture. The

0 same purpose is obtained by increasing the electrode surfaces. It isalso well known to centrally support thediaphragm in order to avoidany,

contact with the counter electrode due to'the great mobility of thediaphragm v In order to comply with the above require- ,ments,diaphragms are made in the known manner from non-conducting material andcoated with a conducting surface of like magnitude as the counterelectrode. An insulating leadingthrough device is passed through theaxis of rotation of the microphonic body-as far as theconductingniaphragm, the latter is then secured to it and the supplyconductor is mounted, This entails the following difliculties: 35 Thegluing of diaphragms requires utmost care since the agglutinant usedruns away most readily' over the insulating border zone and may bringabout not only impairment of the insulation but evendirect-short-circuit of-the electrodes. The comparatively wideinsulating border zone surrounding the conducting part of the diaphragmnecessary for this reason leads a fortiori-to a reduction of the activeelectrode 5 surface and thus to a reduction of the starting pressurereceived from the microphone.

The gluing of the diaphragm presents the disadvantage of the microphonebeing unservice able when in spite of all the precautions taken 50 priorto gluing; minute dust-particles have nevertheless adhered to thecounter electrode etc.

and as a'result'subsequent removal is impossible without destroying thediaphragm.

Cracking of diaphragms involves circuitous e- 65 pairs since a newdiaphragm has to be glued with is rendered non-conductive.

. drawingl in which:

the observance of all conditions involved by manufacture for the firsttime The necessity of supplying the current of the conducting layer tothe diaphragm surface in the center thereof involves a diiiicult finemechanical 6 work and thus'an increase in manufacturing expenses.

These disadvantages are obviated by the present invention. According tothe inventio for consequently preferably at the border with aninsulating border zone. Indeed, it is so constructed that the diaphragmat or adjacent the point, preferably the center, at which is supported 5from. the counter-electrode, is non-conductive or The diaphragm maytherefore be secured directly to the counter-- electrode which for thispurpose has left on it a small projection. Current is supplied to thedia hragm according to the invention in-the most simple manner by thelatter having its conducting layer side mounted directly on the metalcasing or body of the microphone. It ,may be glued thereto'directly butaccording to a further feature of the invention for the sake of itspressure it is preferably glued over a suitably shaped metal ring and isthen only laidon the microphonic body.

This ensures the possibility of readily interchang- 80 mg so thatmounting dimculties are also overcome and dust-particles can always beremoved readily and without expenses.

The small non-conducting surface at the point of support may be made,forexampl'e, by removal 86' mica-disc may be laid on the point concernedso 40 that-this point is kept free from the metal layer. The procedurefollowed may, however, be such that the diaphragm rendered conductingper se throughout its surface is rendered non-conducting at the point ofsupport by painting with an insulating lacquer or the like. Thisprocedure may \be followed, for example, with diaphragms that are onlymade of metal.

In order that the invention may be clearly understood and readilycarried'into effect, one embodiment thereof will-now be describedmoreiully hereinafter with reference to. the accompanying Fig. 1 shows acondenser microphone of the constructionused hitherto, whereas J8 Fig. 2shows a form of construction of the microphone according. to the presentinvention.

'Reierring to Fig. 1, ll designates the microphonic body which hasarranged in it the stationary condenser electrode II. It has centrallyarranged in it the leading-through device. I4 and insulated therefromthe supply conductor l5 ior the movable electrode. In this case, themovable electrode is constituted by a. collodion film l6 glued to theouter edges of the microphonic body ll after prior stressing and havingon its lower side a metallic coating H which, however, only covers theinmart of the diaphragm and the circumference of which is separated'fromthe miprophonic body H by an insulating border zone l8, The accuratefitting oi the insulating leading-through device I and the arrangementof the supply conductor l5 involved extremely great precision andnevertheless ,by reason 01 the microphone being made up from any parts,it did not warrant correct adjustment for long intervals of time.

These diificulties are overcome by the form of construction according tothe invention shown in Fig. 2 in which, as before, the stationary orcentral, electrode 23 is arranged supported in position by an insulatingintermediate layer 22, located in the microphonic body 2|. Instead ofthe insulating leading-through device, this electrode has only in thecenter a small trunnion 24 having a height equal to the distance fromthe diaphragm and being made with advantage in one piece with theelectrode 23. The diaphragm 26 of collodion or other suitable insulationmaterial is provided with the metal coating 21 except,

for a part in its center and glued to a metal ring 25 after priorstressing. This 3 ring 25 permits of the diaphragmbeing readily mountedsecurely but readily removably on the condenser body II which for thispurpose is provided with a ledge 29. The metal coating 21 of thediaphragm is separated by the border zone 28 insulating the diaphragmfrom the trunnion 24 of the stationary electrode. When comparing the twoforms of construction for equal diameters of diaphragm it is obviousthat the space occupied by theborder zone 28, if premised that thewidthsof the border zone are equal, is smaller than the spaceoccupied by thejborder zone in the embodiment hitherto used shown in Fig. 1.

What is claimed is:

1. An electrostatic microphone comprising an outer metallic casinghaving a ledge surrounding the outside surface, an insulating memberwithin said casing,' a central electrode within said insulating member,a projection on said central electrode, a ring seated on said ledge, adiaphragmv secured and electrically connected to said ring,.

diaphragm forming a movable electrode and.

having insulating means for resting on the projection of said fixedelectrode.

3. An electrostatic microphone comprising an outer metallic casinghaving anchoring means on a portion of the outside wall thereof, aninsulating member concentric with the inside wall of said casing, afixed electrode concentrically located within said insulating member, a.small central projection org said fixed electrode, a diaphragm securedbeyond the outer periphery of said casing by said anchoring means andelectrically connected thereto, said diaphragm forming a movableelectrode and having insulating means for resting on the projection ofsaid fixed electrode. 4. An electrostatic microphone comprising an outermetallic casing, an insulating member resting on at least one wall ofsaid casing, a fixed electrode within said insulating member, a centralprojection on said fixed electrode, a metallic diaphragm having anaperture therein, said aperture being covered by an insulating layerwhich rests on said projection, said metallic diaphragm beingelectrically connected to the outer periphery ofsaid casing.

.5. An electrostatic microphone comprising an outer metallic casinghaving anchoring means on a portion of the outside wall thereof, aninsulating member resting on at least one wall of said casing, a fixedelectrode within said insulating member a small projection on said fixedelectrode, a metallic diaphragm having an aperture therein, saidaperture being covered by a film oi non-conducting material which restsonsaid projection, said metallic diaphragm being secured by saidanchoring means and electrically connected to the outer periphery ofsaid casing.

ROLF HORKHEIMER.

